The present invention generally relates to transfer units for use with electrophotographic printers, and more particularly to a control over a transfer current which a transfer unit outputs. The present invention is suitable, for example, for a transfer unit in an electrophotographic printer. A printer according to the present invention is widely applicable not only to a single printer unit but also to those copiers, facsimile machines, computer systems and word processors, and combination machines thereof which have a print function.
An electrophotographic printer typically includes a photosensitive drum and transfer portion. A printing paper passes between the photosensitive drum and transfer portion. The photosensitive drum is charged by a pre-charger, exposed by a laser, and provided with a toner image on its surface by the development. The transfer portion typically utilizes a corona wire to apply a transfer current onto a printing paper using a corona discharge. The transfer portion attracts and attaches the toner image onto the printing paper using the transfer current, thereby completing a transfer of the toner image onto the printing paper.
A transfer current value should be adjusted according to a paper size (width), a printing-paper feed speed, a print environment (such as surrounding temperature and humidity), a printing-paper resistance (which differs according to different paper manufacturers and different paper types, e.g., thick and thin papers), print modes including single-sided (or anopisthographic) and double-sided (or duplex) prints. This is because an excessively small transfer current would insufficiently attract toner onto a printing paper for a desired transfer while an excessively large transfer current would deteriorate a printing quality by an unequal potential distribution.
Conventionally, n signal lines and 2.sup.n current control circuits have determined the transfer current. The current control circuit, as used herein, refers to a circuit that determines a target transfer current value and an essential part in the transfer circuit. Two signal lines, for instance, identify four states, such as, for example, [0, 0], [0, 1], [1,0] and [1,1] in response to digital signals 0 and 1. Information on these signal lines is eventually fed to current control circuits nos. 1 through 4. The current control circuits nos. 1 through 4 respectively store different four types of transfer current values VT1 through VT4. For example, the current control circuit no. 1 corresponds to [0,0], and outputs VT1 as a transfer current for the signal [0,0].
However, there are many parameters necessary to control the transfer current as mentioned above, and a transfer current value to be changed differs every factor. Thus, four (i.e., VT1 through VT4) is insufficient for the number of transfer currents to be generated in the above example. Although it is conceivable to increase the number of signal lines and current control circuits by the number of desired transfer current values to supplement the insufficient number of producible transfer current values, such a configuration would result in the increased wiring number, the difficult packing and/or an expensive printer.